The present invention relates to an ink-jet printing method and apparatus therefor for printing by discharging ink from a printhead onto a print medium.
A printing apparatus such as a printer, copy machine, facsimile or the like is constructed to print dots on a print medium e.g. a sheet of paper or a thin plastic plate or the like, with the use of each of printing elements (nozzles, heating elements, wire and the like), and to form an image consisting of these dots. The printing apparatus of this type can be classified by the printing system, such as the ink-jet printing system, wire-dot printing system, thermal printing system, laser beam printing system and the like. The ink-jet printing system (ink-jet printer) is structured to discharge ink drops (printing liquid) from orifices of a printhead to a print medium, thereby printing an image.
Along with the recent growth in the number of printing apparatuses used as output terminals of personal computers or image processing apparatuses, demands are high for achieving quick printing, high resolution, high image quality, low noise and so on. The aforementioned ink-jet printer has been provided to satisfy such demands. Since the ink-jet printer performs printing by discharging ink from a printhead, it is possible to realize printing without contacting the print medium, making it possible to stabilize image quality.
Owing to the recent development in various digital cameras, digital videos, CD-ROM or the like, pictorial image data can be readily processed utilizing an application program on a host computer. As a result, printers serving as output units of such computers are required to have the capability to output pictorial images as well. Conventionally, outputting operation of a pictorial image was performed by a printing apparatus adopting the silver salt printing system which is an advanced printing apparatus for inputting a digital image, or sublimation printing system which is an expensive printing apparatus dedicated to output photographs using sublimation dye.
Such printing apparatus used conventionally to print photographic images or the like was extremely expensive. One of the reasons is in that the silver salt printing system requires extremely complicated processes and that the apparatus was so large that it cannot be used as a desk-top device. Also with respect to the apparatus using sublimation dye, as the size of a print medium becomes large, the cost for the main unit and running cost become extremely high. Therefore, individuals could not use these apparatuses with ease. In addition, the largest disadvantage of these apparatuses is that these apparatuses are designed to use a particular print medium. In other words, the type of a print medium a user can use is limited. Since a regular sheet of paper is normally used to print documents or graphics or the like in a personal home or general business environment, color photographic images are difficult to print since they require a special sheet to print pictorially.
The ink-jet printer is known as a printing apparatus that minimizes such limitation related to a print medium. In order to cope with the disadvantage, the ink-jet printer has lately provided a type that can print a color photographic image with great improvement in its image quality, by improving image processes, colorant and print medium or the like.
In addition, to improve tonality of color graphics in a color image output, various techniques have been suggested. Various improvements have been suggested and made available in late years. For instance, resolution is relatively increased from a normal print mode to improve drawing capability, or resolution of a printing apparatus is increased to send multi-value image data as print data to a printing apparatus, to print multi-value image utilizing sub-pixels.
Furthermore, a printing method realized by changing the amount of ink discharged from a printhead is available. According to this method, the amount of ink discharge is uniformly and relatively decreased in a high-resolution mode. Moreover, a printhead which can arbitrarily modulate the amount of ink discharged from each nozzle has been suggested.
However, the above-described conventional method raises the following problems.
In the printing method of uniformly decreasing the discharging amount of ink, printing is performed with increased resolution in each of the main-scanning direction and sub-scanning direction. Therefore, the number of times of scanning in the main-scanning direction increases and the distance that a print medium is conveyed in the sub-scanning direction decreases per one scanning. Accordingly, print speed largely declines. In addition, the amount of data increases as the resolution of print data is raised, resulting in great increase in memory capacity for storing the print data, an increased amount of transfer data and transfer time at an interface, and an increased load in a printer driver. For instance, if resolution of the print data is raised twice as high, the amount of print data increases twice as much in both the main-scanning and sub-scanning directions, resulting to increase the amount of data to a square of two, i.e. four times as much. Moreover, in terms of an outputted image, a diameter of a dot in the image is reduced to a minute size in order to decrease granularity (sense of roughness) in a dark portion of the image. Thus, even in an image portion having a light color where granularity is inconspicuous, a large number of the fine dots are similarly printed. As a whole, printing efficiency is poor despite the improved image quality.
Another method of printing is to utilize mixture of large dots having long diameter and small dots having short diameter. According to this method, poor print efficiency in image forming can be improved. This method is feasible in a case where a single nozzle is provided for each color; however, in a case where a plurality of nozzles are provided for each color, it becomes difficult to realize, and the larger the number of nozzles, the more difficult to realize. Normally, each nozzle discharges ink drops with frequency of several KHz or more. If the number of nozzles is small, the ink discharge operation can be controlled directly by a CPU. However, as the number of nozzles increases, the ink discharge needs to be controlled in terms of processing speed by hardware along with hardware circuits such as a gate array or the like. Note that in order to modulate the discharging amount of ink by making use of large dots and small dots, a driving pulse for discharge is modulated, or plural driving elements for large dots and small dots are provided for each nozzle to be driven alternatively.
To switch the plural driving elements as in the latter case, registers must be provided for printheads for large dots and small dots respectively. In this case, the necessary number of registers is integral multiples of the printing resolution. Therefore, the scale of the printhead circuit becomes large and cost for a printhead becomes high. Meanwhile, to modulate a driving pulse as in the former case, separate driving signal lines are necessary for controlling driving operation of each of the nozzles. Therefore, one signal line which is normally sufficient for driving nozzles would be increased to several hundreds lines (corresponding to the number of nozzles). As a result, the number of contacts (contact point), the number of flexible cables connected to printheads, the number of driver transistors for printing elements also increase, causing cost increase by a great deal.
Instead of achieving printing with the mixture of large dots and small dots by one scanning of a printhead, printing operation can be realized by plural scanning operation, combining scanning for printing large dots and scanning for printing small dots. According to this method, it is possible to generate an image having the mixture of large dots and small dots with a simple construction. However, this method always requires plural times of scanning operation (hereinafter referred to as multiple pass printing). For instance, even if small dots are printed for almost all addresses in single scanning and there is only one large dot in the single scanning line, scanning operation must be performed twice to print that one large dot. In addition, as the number of times of multiple pass printing increases, the printing requires longer time in reality. Therefore, the number of times of multiple pass printing must be minimized. However, a problem arises herein. Assume that printing operation is performed in two pass to form gradation ranging from a low-density portion (thin: white) to a high-density portion (thick). Herein, it is assumed that printing starts from the low-density portion. The portion where a color (including gray scale) starts to appear is printed with the smallest dots. Then as it comes to a portion having a higher density in the image, a number of small dots are printed by the printhead on a printable grid point (virtual print-dot position). When all the small dots are printed in the above manner, the mixture of small dots and large dots are then used to print the image. As it comes to a portion having even higher density in the image, large dots are further printed to form the highest density portion.
Herein, print control in the printing apparatus is executed by switching between the printing operation with large dots and the printing operation with small dots for each scanning operation. When printing is performed in such condition described above, a problem occurs in that, in a case where small dots are printed in all the printable grid points, meaningless scanning must be executed for printing a large dot even if there is no large dot to be printed. In addition, when 100% of the printing processing is executed by one of two scanning (small dots), the advantage of divided printing, that is, the characteristic of multiple pass printing which enables to solve the problems such as an uneven amount of ink discharged from a nozzle or an unequal amount of paper conveyance, cannot be attained. Furthermore, since the ratio of printing amount is not equal for each scanning operation, problems arise in that an error rate is not decreased in scanning operation having a high printing ratio, or that electricity consumption is not reduced since the scanning operation having a high printing ratio consumes high instantaneous electricity.